Microbiologically Induced Corrosion of UNS N04400 in Seawater^☆

Laboratory and field immersion tests were conducted on UNS N04400 (Monel^† 400) to assess its susceptibility toward microbial attack in Arabian Gulf seawater. Specimens were exposed to chlorinated and nonchlorinated seawater for periods up to four months. Other tests included exposure of UNS N04400 to a prepared culture of sulfate-reducing bacteria (SRB). The results indicated that SRB attack initiated beneath a black sulfide-rich deposit, overlaid by a green chloride-rich deposit, and a beige-colored calcareous scale was on all specimen surfaces after long exposure periods. The black layer was comprised mainly of one or more iron-nickel sulfides (Fe, Ni)_xS_y and nickel sulfide (Ni₃S₄), whereas the green layer was essentially CuCl₂ x 3Cu(OH)₂ with a low concentration of NiCl₂. These deposits were in the form of nodules scattered on the surface, and, when removed, circular attacked areas or cavities were revealed. Scanning electron microscopy examinations of these areas indicated severe intergranular corrosion, and energy dispersive spectroscopy indicated selective dissolution of Ni and Fe.